The present invention is directed to bicycle transmissions and, more particularly, to bicycle transmissions mounted inside a wheel hub.
Conventional bicycle transmissions can be divided into two types: transmissions that utilize a derailed that is engaged with a chain and that aligns the chain with one of a plurality of gears mounted to a crank or rear wheel of the bicycle, and internal transmissions that are installed in the wheel hub. The apparatus disclosed in Japanese Laid-Open Utility Model Application 57-42792 is an example of an internal transmission. An internal transmission basically makes use of a planet gear mechanism to provide a plurality of shift steps.
The structure of an internal transmission will be described briefly at this point. The important parts in an internal transmission are the fixed shaft that is fixed to the fork of the bicycle, the driver that is rotatably supported on this fixed shaft by bearings or the like and that transmits the drive force from the chain via a gear, and a hub shell that transmits the drive force from the driver via a plurality of drive force transmission routes. The rear wheel is supported on this hub shell via spokes or the like. A planet gear mechanism that forms the plurality of drive force transmission routes is located between the driver and the hub shell. The planet gear mechanism has a sun gear that is provided to the fixed shaft and a planet gear that engages with this sun gear. The planet gear is usually an annular member provided with gear teeth on its outer surface, and it is designed such that it rotates while it revolves with respect to the fixed shaft by means of a gear frame rotatably supported by the fixed shaft. A ring gear that engages with the teeth of the planet gear is often provided radially outwardly from the planet gear. The transmission path through the planet gear mechanism is selected by a clutch that is operated by the rider.
When the bicycle is pedaled, the drive force is transmitted to the driver via the chain and the gear engaged with the chain. The drive force from the driver is transmitted to the planet gear via the gear frame, and when the auto-rotation of this planet gear is transmitted to the hub shell that supports the wheel, the rotation of the driver is accelerated as it is transmitted to the hub shell. When the drive force from the driver is transmitted to the planet gear via the ring gear and is transmitted from the planet gear to the hub shell through the gear frame and the like, the rotation from the driver is decelerated as it is transmitted to the hub shell. Because the clutch must change the path of meshing gears within the planet gear mechanism, a relatively large operating force is sometimes required to operate the clutch. This problem is particularly noticeable when the drive load is heavy, such as when the bicycle is being pedaled hard.